[zlib][build] Restrict third_party/zlib:zlib_common_headers visibility

That target is only used in //third_party/zlib:*, so make that explicit.

From documentation
(https://gn.googlesource.com/gn/+/master/docs/reference.md#var_visibility)
this change will make the given target 'private' only to the current
build file.

No expected change in behavior, but a nice cleanup.

Change-Id: I0bc111bd7bda05b450f7d1b82aebbd5bdc746b05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3672796


Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1009027}
NOKEYCHECK=True
GitOrigin-RevId: f2c70a1264eff5f762bf97d2c50e15116c2633dc